Photomasks, also called masks, are used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide wafer. In general, a photomask is comprised of a transparent substrate to which a masking material layer is affixed and patterned. The pattern of the masking material is a scaled master of the image desired to be formed on the semiconductor wafer.
The transfer the photomask image to the semiconductor wafer occurs through a process commonly referred to as photolithography. More specifically, a wafer exposure system is used to interpose the photomask between a semiconductor wafer which is coated with a layer of photosensitive material and an optical energy source. Energy from the wafer exposure system is inhibited from passing through the areas of the photomask in which the masking material is present. However, energy generated by the water exposure system passes through the portions of the substrate of the photomask not covered by the masking material and causes a reaction in the photosensitive material on the semiconductor wafer. Through subsequent processing, the image created on the photosensitive material is transferred to the semiconductor wafer.
Since the masking image on the photomask directly correlates to the image created in the semiconductor wafer, any foreign substance or contamination on the surface of the mask during the photolithographic process will cause unwanted images of these artifacts to be printed on the semiconductor wafer. To reduce or eliminate photomask surface contamination, a thin, transparent membrane or film commonly referred to as a pellicle is stretched across an anodized aluminum frame mounted on the photomask before the photolithographic process is begun.
FIGS. 1A and 1B depict a top and side view of a typical photomask configured for use in the photolithographic process. As shown, photomask 2 (typically six inches by six inches in size and one-quarter inch thick) is comprised of transparent substrate 4 (e.g., fused silica) and the pattern layer of masking material 6 (e.g., chromium) defining the desired image to be created on the semiconductor wafer. Pellicle frame 8 extends around the perimeter of the patterned masking material 6 and is affixed to the substrate 4 via vapor deposition as well known in the art. Pellicle membrane 10 is stretched over and affixed to the upper surface of frame 8. As shown, the surface of pellicle membrane 10 is generally parallel to the surface of the photomask and covers the entire patterned area of masking material 6. Thus, any contamination which would otherwise land on the photomask instead falls on the pellicle membrane 10 staying out of the wafer exposure system focal plane.
Pellicle membranes known in the prior art are made of organic material such as nitocellulose or other fluorocarbon based polymers. Non-uniformities in transmission and birefringence caused by pellicle membranes result in pattern fidelity errors which become more prevalent when feature sizes patterned into the semiconductor wafer are in the sub-wavelength regime and may ultimately result in diminished device performance or failure.
The prior art pellicle membranes are susceptible to being scratched and torn, and any damage to the thin pellicle membrane requires the entire pellicle to be removed and replaced. Of course, during the time the pellicle membrane is being removed and replaced, the photomask cannot be used for semiconductor fabrication. Additionally, the extensive rework procedure required to remove and replace damaged pellicles sometimes results in the ultimate rejection of the entire photomask. Further, as discussed above, the pellicle membrane 10 prevents contaminants from reaching the photomask surface and therefore must be cleaned occasionally. Pellicles are typically cleaned using a nitrogen gun. However, due to their somewhat fragile nature, the prior art pellicle membranes have a propensity to break or otherwise become damaged during the cleaning process requiring their removal and replacement. Also, defects that cannot be removed with a nitrogen gun also cannot be removed mechanically for fear or scratching or tearing the membrane. Here again, during the pellicle replacement process, the photomasks cannot be used for semiconductor fabrication and there is a risk of rejection of the entire photomask
Accordingly, it is the object of the present invention to overcome the shortcoming of the prior art by providing a pellicle for use on a photomask having improved uniformity of transmission and birefringence thereby increasing pattern fidelity.
It is a further object of the present invention to provide a pellicle which is less susceptible to damage and therefore can be easily cleaned.
It is a further object of the present invention to provide a reusable pellicle which can be easily removed, cleaned, and re-installed on a photomask.